Introduction to Gastrointestinal Anatomy & Physiology

The gastrointestinal (GI) system is one of the most vital and intricate systems in the human body, responsible for processing the food and nutrients necessary to sustain life. From the moment food enters the mouth to its eventual elimination as waste, the GI system performs a remarkable series of mechanical, chemical, and enzymatic processes to break down and absorb nutrients. In this section, we will explore the intricate structure and function of the gastrointestinal (GI) system, focusing on how it processes nutrients and eliminates waste. The GI system is a complex network of organs, glands, and tissues that work together to carry out motility, secretion, digestion, and absorption. By the end of this section, you'll have a deeper understanding of how each component contributes to maintaining homeostasis and overall health.

Functions of the GI System

The primary functions of the GI system revolve around motility, secretion, digestion, and absorption:

1. Motility: Moves food through the GI tract via coordinated muscle contractions.

2. Secretion: Involves the release of enzymes, bile, and hormones necessary for digestion.

3. Digestion: Breaks down food into smaller, absorbable components.

4. Absorption: Transfers nutrients into the bloodstream or lymph for distribution.

Sequence of Digestive Events

1. Chewing (Mastication): Breaks down food while saliva lubricates and forms a bolus.

2. Stomach Mixing and Grinding: Gastric acid dissolves and denatures food components.

3. Accessory Organ Secretions: Bile from the gallbladder and enzymes from the pancreas are released.

4. Small Intestine Transit: Food moves as chyme over 7-10 hours, where most absorption occurs.

5. Large Intestine Transit: Takes 12-24 hours as water and electrolytes are absorbed.

Nutrient Absorption in the Small Intestine

The small intestine absorbs nutrients through its three sections:

• Duodenum: Absorbs iron and calcium.

• Jejunum: Responsible for carbohydrate, protein, and fat absorption.

• Ileum: Specializes in absorbing bile acids and vitamin B12.

Autonomic Nervous System and GI Innervation

The GI tract is regulated by the autonomic nervous system (ANS):

• Parasympathetic (Rest and Digest): Enhances motility, secretion, and absorption via vagus and pelvic nerves.

• Sympathetic (Fight or Flight): Reduces motility and shuts down digestive functions during stress.

What in the world does all of this mean? Here's a fun example to explain it further, and if you don't understand now it's okay! Don't panic! We will cover more about this in the nervous system section, but here's a sneak peak:

Fight, Flight, or Freeze: The Bear Encounter Breakdown

Imagine you’re hiking in the woods, enjoying nature, when suddenly… a bear appears! 🐻 Your nervous system immediately kicks into action, and what happens next depends on which part of the nervous system takes charge. Let’s break it down:

1. Sympathetic Nervous System: The "Fight or Flight" Hero

This is your emergency response system—the one that says, “We’re in trouble, act now!”

• Heart: Boom, boom, boom! Your heart starts racing like you're sprinting through a marathon. Why? To pump more blood to your muscles so you can run like Usain Bolt or throw a punch like Rocky.

• Breathing: Deep and fast! Your lungs expand, pulling in more oxygen to fuel your escape.

• Digestion: Nope, no time for snacks! The sympathetic system shuts down your digestion because outrunning a bear is more important than digesting that granola bar.

So, if you decide to run away or fight the bear, thank your sympathetic system for getting you pumped up.

2. Parasympathetic Nervous System: The "Rest and Digest" Peacekeeper

Now imagine you’ve escaped the bear (whew!) and find a quiet place to hide. Your parasympathetic system takes over, calming everything down:

• Heart Rate: Slows down. Your heart goes from thud-thud-thud to a gentle bump-bump-bump.

• Breathing: Deep sigh of relief. Your lungs no longer feel like they’re about to explode.

• Digestion: Your stomach starts growling again, reminding you that granola bar is still waiting to be eaten.

The parasympathetic system is your body’s way of saying, “Crisis over, time to chill.”

3. The Aftermath: Adrenaline and the Crash

Once the bear wanders off (phew!), you might feel shaky or exhausted. That’s the adrenaline wearing off as your parasympathetic system restores balance.

Motility: Voluntary vs. Involuntary Muscle Movement

Did you know that some GI functions are under your control, while others happen automatically? This balance between voluntary and involuntary muscle movement is crucial to digestion.

For example, the lower esophageal sphincter (LES)—the muscle separating your esophagus from your stomach—contracts involuntarily. When this doesn’t work as it should, you might experience achalasia, where the LES fails to relax properly, causing difficulty swallowing.

On the flip side, if the LES becomes too loose, it can lead to GERD (gastroesophageal reflux disease). This common condition allows stomach acid to splash back into the esophagus, causing that infamous burning sensation known as acid reflux.

The Stomach: Wrinkles and Rugae

The stomach isn’t just a simple pouch; it’s equipped with rugae, which are folds in its lining. These wrinkles allow the stomach to stretch when you overindulge at your favorite buffet 🍴. Think of rugae as the stomach’s elastic waistband, accommodating extra food while it stores and digests the contents before passing them on to the duodenum (the first part of the small intestine).

Clinical Importance of the Small Intestine

The small intestine is essential for nutrient absorption, digestion, and maintaining overall metabolic balance. It is composed of three sections: the duodenum, jejunum, and ileum, each serving specific roles in nutrient absorption.

• Duodenum: The first part of the small intestine where the majority of digestion occurs. Digestive enzymes from the pancreas and bile from the liver enter here to help break down fats, proteins, and carbohydrates. The duodenum is also responsible for the absorption of iron and calcium.

• Jejunum: The middle section, which is the primary site for absorption of most nutrients, including carbohydrates, proteins, and fats. The jejunum is lined with villi that help increase surface area for absorption.

• Ileum: The final section responsible for absorbing vitamin B12 and bile acids, which are then recycled back to the liver for reuse in digestion.

Clinical Relevance:

• Diseases such as Celiac disease and Crohn’s disease can disrupt absorption in the small intestine, leading to malnutrition and deficiencies in nutrients.

• Celiac disease involves an autoimmune response triggered by gluten, leading to damage of the villi in the small intestine, impairing nutrient absorption.

• Crohn’s disease can cause inflammation and ulcers in any part of the digestive tract but commonly affects the ileum, leading to malabsorption and nutrient deficiencies.

Vomiting: A Built-in Emergency Eject System

Vomiting is the body's way of protecting itself from harmful substances. It can be triggered by: 🤮

• Noxious stimuli in the gut (e.g., bad food)

• Signals from the brain, such as nausea caused by a foul odor or motion sickness

• Circulating chemicals, like certain drugs or hormones during pregnancy

The central nervous system vomiting center coordinates this complex process, which involves reverse peristalsis (waves of contraction moving backward) in the stomach and small intestine, as well as relaxing the sphincters to allow the contents to exit.

Gastric Acid Secretion: What Goes Wrong in GERD

The stomach produces gastric acid to aid in digestion, but it also has a feedback system to prevent overproduction. When acid levels increase, the hormone somatostatin steps in to slow things down. If this mechanism isn’t working properly, medications like omeprazole—a proton pump inhibitor (PPI)—are often used to block the production of new acid.

Clinical Tip: PPIs work best when taken preventively, not just when symptoms flare, because they stop the production of new acid rather than neutralizing existing acid.

The Pancreas: The Unsung Hero of Digestion

The pancreas plays two critical roles I'll bet you didn't even know about:

1. Digestive enzymes: It secretes enzymes to break down proteins, fats, and carbohydrates into absorbable nutrients.

2. Neutralizing acid: It releases an alkaline fluid to neutralize stomach acid as it enters the small intestine.

Yes, I know, it also has an endocrine function, producing hormones like insulin and glucagon (we’ll dive deeper into this in the Endocrine chapter).

Intestinal Absorption: Water and Nutrients

Most absorption occurs in the small intestine, but the large intestine (colon) also plays a key role, especially in water absorption:

• The right colon absorbs the majority of water.

• The left colon stores waste before it’s eliminated.

Each day, 7–10 liters of water pass through the small intestine, but only 1–3 liters reach the colon.

Bilirubin and Jaundice

Bilirubin, a byproduct of red blood cell breakdown, is processed in the liver and excreted in bile. When this process is disrupted, jaundice can occur, causing yellowing of the skin and eyes. This is common in liver disease or in newborns with underdeveloped livers.

Liver Metabolism: A Powerhouse of Activity

The liver is metabolically active, regulating:

• Glucose: Through processes like gluconeogenesis and glycogen storage.

• Proteins and fats: Synthesizing and breaking them down as needed.

• Vitamin storage: Holding reserves of fat-soluble vitamins A, D, E, and K.

While these processes can get complex, just remember: the liver is essential for maintaining metabolic balance and nutrient storage.

Common GI Pathologies

a. Peptic Ulcer Disease (PUD)

• Pathophysiology: PUD is the result of an imbalance between stomach acid and the protective mechanisms of the stomach lining. The ulcers can form in the stomach or duodenum and are commonly caused by an infection from Helicobacter pylori or the use of nonsteroidal anti-inflammatory drugs (NSAIDs).

• Symptoms: Burning stomach pain, bloating, nausea, vomiting, and, in severe cases, gastrointestinal bleeding.

• Treatment: Antibiotics to eradicate H. pylori, proton pump inhibitors (PPIs) to reduce acid production, and avoidance of NSAIDs. In severe cases, surgery may be required.

b. Diverticulitis

• Pathophysiology: Inflammation or infection of pouches (diverticula) that form in the walls of the colon. The condition is most common in older adults.

• Symptoms: Abdominal pain (usually on the left side), fever, nausea, and changes in bowel habits.

• Treatment: Mild cases are treated with antibiotics and a clear liquid diet. Severe cases may require hospitalization or surgery to remove the affected portion of the colon.

c. Irritable Bowel Syndrome (IBS)

• Pathophysiology: A functional gastrointestinal disorder that affects bowel motility without underlying structural abnormalities. Stress and diet are often triggers.

• Symptoms: Chronic abdominal pain, bloating, diarrhea, and constipation.

• Treatment: Dietary changes (low FODMAP diet), stress management, and medications (antispasmodics, fiber supplements, or laxatives).

d. Lactose Intolerance

• Pathophysiology: Inability to properly digest lactose, a sugar found in milk and dairy products, due to a deficiency in lactase enzyme production.

• Symptoms: Diarrhea, bloating, and gas after consuming dairy products.

• Treatment: Avoidance of dairy products or use of lactase supplements to aid in digestion.

Accessory Organ Diseases

a. Gallstones (Cholelithiasis)

• Pathophysiology: Hard deposits form in the gallbladder, blocking the bile ducts and causing pain, inflammation, and infection. Gallstones can be cholesterol-based or pigment-based.

• Symptoms: Sudden, severe pain in the upper right abdomen, nausea, and vomiting.

• Treatment: Small stones may be treated with medications, but larger or symptomatic stones typically require surgery (cholecystectomy) to remove the gallbladder.

b. Pancreatitis

• Pathophysiology: Inflammation of the pancreas, often due to alcohol use or gallstones. The pancreas becomes inflamed and its enzymes start digesting its own tissue.

• Symptoms: Severe upper abdominal pain, nausea, vomiting, and fever.

• Treatment: Hospitalization with IV fluids, fasting from food to allow the pancreas to heal, and treatment of underlying causes (e.g., gallstone removal or alcohol cessation).

c. Hepatitis

• Pathophysiology: Inflammation of the liver, which can be caused by viral infections (hepatitis A, B, C), alcohol abuse, or autoimmune conditions.

• Symptoms: Jaundice (yellowing of the skin/eyes), fatigue, nausea, and abdominal pain.

• Treatment: Antiviral medications for chronic hepatitis B and C, alcohol abstinence, and liver transplantation in severe cases.

Colon Cancer Screening and Prevention

Prevalence:

• Colorectal cancer is the third most common cancer in the U.S. and the second leading cause of cancer deaths. The American Cancer Society recommends that people at average risk begin screening at age 45.

Screening Methods:

• Colonoscopy: Recommended every 10 years for individuals at average risk. It allows for direct visualization of the colon and rectum and enables removal of precancerous polyps.

• Fecal Immunochemical Test (FIT): A non-invasive, stool-based test that can detect blood in the stool, which may indicate colon cancer.

• Cologuard: An at-home, mail-in test that detects both blood and genetic markers in stool to assess for colon cancer risk. Recommended every 3 years.

Risk Factors:

• Age (over 50 years)

• Family history of colorectal cancer or polyps

• Inflammatory bowel disease (e.g., Crohn’s disease, ulcerative colitis)

• Diet (high in red/processed meats and low in fiber)

• Sedentary lifestyle, smoking, and alcohol use.

Prevention:

• Regular screening, a high-fiber, low-fat diet, and regular exercise are key to preventing colorectal cancer. Removing polyps during a colonoscopy can prevent cancer from developing.

G.I. Emergencies

a. Bowel Obstruction

• Diagnosis: X-rays or CT scans to identify blockage, abdominal ultrasound to check for signs of bowel distention.

• Symptoms: Severe abdominal pain, vomiting, inability to pass stool or gas, and distended abdomen.

• Treatment: Hospitalization, IV fluids, and nasogastric tube (NG tube) placement to relieve pressure. Surgery may be required if the obstruction is caused by a physical blockage like a tumor or adhesions.

b. Perforated Ulcer

• Diagnosis: X-ray or CT scan to detect free air in the abdomen (sign of perforation).

• Symptoms: Sudden, severe abdominal pain, nausea, vomiting, and signs of peritonitis (e.g., abdominal rigidity).

• Treatment: Emergency surgery to repair the perforation and prevent peritonitis. IV antibiotics and proton pump inhibitors (PPIs) are used to control stomach acid production.

c. GI Bleeding (Upper vs. Lower)

• Diagnosis: Endoscopy for upper GI bleeds (esophagus, stomach, duodenum), colonoscopy for lower GI bleeds (colon, rectum).

• Symptoms: Vomiting blood (hematemesis), black or tarry stools (melena), bright red blood in stool (rectal bleeding).

• Treatment: Blood transfusions, medications to reduce stomach acid (e.g., PPIs), and possibly endoscopic procedures to stop bleeding (e.g., cauterization of bleeding vessels).

Bariatric Surgeries: Roux-en-Y Gastric Bypass & Sleeve Gastrectomy

Vitamin Absorption:

• While Sleeve Gastrectomy is less invasive than Roux-en-Y Gastric Bypass and does not involve rerouting the intestines, it still affects the absorption of certain nutrients, although to a lesser extent.

  • B12 absorption: The reduced stomach size and altered digestive process can lead to B12 deficiency over time. B12 is primarily absorbed in the stomach and the small intestine. With a reduced stomach capacity, there is less secretion of intrinsic factor, which is necessary for vitamin B12 absorption.

  • Iron and calcium absorption: As with B12, the absorption of iron and calcium can also be impaired due to reduced stomach size and decreased acidity, which is necessary for the optimal absorption of these minerals.

  • Other vitamins: Fat-soluble vitamins (A, D, E, and K) may also have impaired absorption due to a reduction in food intake and altered gastrointestinal motility.

• Supplementation: Patients who undergo SG are typically required to take lifelong vitamin and mineral supplements to prevent deficiencies. Common supplements include:

  • Multivitamins (especially those containing B12, folate, and iron)

  • Vitamin B12 (in oral or sublingual form)

  • Iron supplements

  • Calcium citrate (preferably in chewable form, as calcium carbonate may not be well absorbed)

Indications for Surgery:

• Severe Obesity: SG is recommended for patients with a BMI ≥ 40 or a BMI ≥ 35 with obesity-related comorbidities (such as type 2 diabetes, hypertension, sleep apnea, and heart disease).

• Failure of Conservative Weight Loss Methods: Patients who have been unsuccessful with traditional weight loss methods (diet and exercise) and have been evaluated by a multidisciplinary team, including surgeons, dietitians, and psychologists, may be candidates for surgery.

• Age Range: Typically, SG is considered for adults aged 18-65, although in some cases, younger patients may qualify, particularly those with significant obesity-related health problems.

Risks:

• Nutritional Deficiencies: Because of the reduced stomach size and changes in the digestive process, patients are at risk for deficiencies in vitamins and minerals, especially B12, iron, and calcium.

• Gastric Leakage: A possible complication where the staple line (the area where the stomach was stapled) leaks, potentially causing infection and requiring reoperation.

• Blood Clots: Like any surgery, there is a risk of blood clots forming, particularly in the legs, which can travel to the lungs (pulmonary embolism).

• Gastroesophageal Reflux Disease (GERD): SG may exacerbate or cause GERD in some patients, leading to heartburn or regurgitation.

• Dumping Syndrome: While less common than in Roux-en-Y gastric bypass, some patients may experience dumping syndrome, especially if they consume high-sugar foods. Symptoms include nausea, vomiting, sweating, and dizziness after eating.

• Stomach Stretching: Over time, the stomach may stretch slightly, allowing patients to eat larger meals. This can slow or reduce weight loss if dietary habits are not closely monitored.

• Long-Term Risks: Chronic vitamin deficiencies and malnutrition are long-term risks, necessitating regular monitoring of vitamin and mineral levels through blood tests.

Follow-Up Care:

• Regular Blood Tests: Patients will need annual or bi-annual follow-up visits with their healthcare provider to monitor for nutritional deficiencies. These tests typically include:

  • Vitamin B12 levels

  • Iron and ferritin levels

  • Calcium levels

  • Vitamin D levels

  • Complete blood count (CBC) to monitor for anemia

• Dietary Monitoring: Patients will work with a dietitian to ensure they are meeting nutritional needs and avoiding potential deficiencies.

• Psychological Support: Long-term weight management also requires psychological support to address the emotional and behavioral aspects of eating.